When a body of aluminum is melted in a furnace for purposes of recovering aluminum from ore or the like, dross forms on the surface of the molten aluminum and must be periodically removed, for example by skimming or similar operation. The removed dross generally contains substantial amounts of free aluminum as well as aluminum oxides and certain other metals and metal salts of magnesium, manganese, and lithium, depending on the nature of the aluminum or aluminum alloy being treated. It is desirable to recover from the aluminum dross to the extent possible the free aluminum since this free aluminum comprises substantial economic value.
In one conventional way of treating the dross to recover free aluminum, a salt such as sodium chloride is used to blanket the molten dross. The free aluminum is then separated from the dross, providing free aluminum and a non-metallic product containing aluminum oxides and other materials such as aluminum nitrides, aluminum chlorides and aluminum carbides, as well as substantial amounts of salt. The entire non-metallic materials recovered from the dross treatment are referred to herein as non-metallic products (NMPs). These non-metallic products, to a substantial part using the conventional salt treatment, are considered waste materials and conventionally are disposed of in landfills. However, these non-metallic products, since they contain salt and aluminum nitrides (AlN), cannot be disposed of in conventional landfills without treatment since the salt can be leached out during rainfall and the aluminum nitrides are reactive when in contact with moisture, producing ammonium. As a result of this reactivity and ecological considerations, it has been necessary before carrying the NMPs to landfills to remove the salt and aluminum nitrides from the NMPs recovered from salt cake. This has been conventionally done utilizing a desalinization plant wherein the NMPs from salt cake are exhaustively washed and treated to remove salt and the aluminum nitrides to as low a level as possible, preferably below less than one percent. The treatment is costly, and additionally landfills are becoming scarce.
It has been found according to an invention described in application Ser. No. 07/902,025 filed Jun. 22, 1992 that it is advantageous not to treat the NMPs from salt cake so as to remove the aluminum nitrides. Rather, it has been found that non-metallic products containing aluminum nitrides (AlN) can be milled and screened and then advantageously utilized to produce useful refractory, abrasive and the like articles. The AlN within the non-metallic product are reactive in processes converting the non-metallic product into useful refractories and the like articles to provide heat energy useful in forming the desired refractories and the like articles. The aforesaid invention, therefore, not only eliminates the need for the economically disadvantageous exhaustive desalinization treatment of the dross but also provides a useful source of raw material, eliminating the need to put the material into a landfill.
The aforesaid invention, therefore, provides a process wherein a desalinization facility is associated with a dross treatment operation, designed and constructed to wash or mill salt cake whereby the salt levels are reduced by such operations to about 3% or less but where the AlN levels are preserved as much as possible, preferably above about 7% and more preferably in the 12%-24% range. The NMP so treated will also contain free aluminum.
In operations which do not use salt in the treatment of the ore, salt-free dross or "white dross" is recovered. White dross containing high levels of aluminum nitride can be treated directly in a plasma energy furnace as disclosed in U.S. Pat. No. 4,877,448, 4,997,476 and 5,030,273 to recover free aluminum and valuable non-metallic products containing reactive aluminum nitride with the AlN levels being in the range of 7-24% and free aluminum being in the range of 3-7%.
The refractory NMPs having high levels of free aluminum and AlN have application in providing useful refractory materials. For example, U.S. Pat. No. 5,132,246 describes a process for producing a refractory product from NMPs containing aluminum nitride without prior conversion of the aluminum nitride in the residue to aluminum oxide or aluminum hydroxide by mixing the NMPs with a material selected from the group consisting of magnesium oxide, silicon oxide, calcium oxide, nickel oxide, titanium oxide and precursors thereof and then heating the resultant mixture at a temperature in the range of about 1000.degree. to 2300.degree. C. to produce a refractory product. The aforesaid '246 patent does not describe how the mixing of the reactive NMP is to occur other then simply stating that it is accomplished in a crucible.
The NMPs obtained either according to the invention described in Ser. No. 07/902,025, or in a plasma furnace, or by other procedures has a very fine particle size, i.e., in the range of 0.3 to 300 microns. In conventional practice, in order to commercially process these fines in a calcination and/or sintering process, it is necessary to first process the fines through agglomerization prior to treatment as, for example, in a kiln. Such a process is described in U.S. Pat. No. 4,523,949. Attempts to directly process these fines in a kiln leave the fines entrained in the exhaust gas stream. These entrained fines have low residence time within the kiln, burden the exhaust stream and/or are poorly treated. At best, these fines remain fines, i.e., with a minimum quantity of larger aggregates being formed. Fines agglomerization equipment is expensive to operate and maintain, requiring an extra processing step such as briquetting, compacting, granulating or the like. Such agglomerization handling can result in emission of unacceptable fumes into the atmosphere also causing ecological problems.
Fines are generated by refractory, metallurgical, and glass industries and are collected by bag houses and electrostatic precipatators. These fines are often land filled as wastes or agglomerated prior to thermal processing in wet process granulators. These agglomerates generate a high percentage of fines on thermal treatment and much of the agglomerate end up back in the bag house or electrostatic precipatator.
Accordingly, there is a need for a process for treating small particle NMP fines and other fines such as electrostatic precipatator (ESP) dusts and bag house dusts in a high temperature atmosphere in order to produce aggregate materials on a continuous basis without having to first agglomerate the fines.